Well, that's pretty easy. Isekai used to always be "
Voyage and Return" stories. A person (usually a teenager) is thrown into a new and wondrous world, then discovers danger in this world. He has to overcome this danger and various other challenges, in the process developing as a human and growing up. He then returns home to the mundane world, having been changed by the experience.
The point is about the main character being changed by the experience. It represents a yearning to grow and develop through challenges that are clearer and more easily confronted than the challenges in the real world. (ie A monster to destroy instead of homework to do).
The challenges are supposed to be hard and scary and the character has to struggle to overcome them - and then he returns, having matured for the better. Basically it's a yearning for an adventure so that you can grow up.
Chronicles of Narnia, Alice and Wonderland, Inuyasha, all sorts of stories fit this mold.
Nowadays though kids don't want to mature and live in the real world. The want to escape the world they live in. So isekai stories now almost never involve wanting to return to the new world, because who would want to go back to the real world? Most also don't involve maturing or overcoming difficulties. Instead you get characters who show up in a new world, and they got some kind of cheat ability (even if it's just knowledge from a previous life) that suddenly elevates them to mastery level. Then they use this cheat ability to easily gain wealth, power, women, or whatever it is that fills their appetite. Yearning for an adventure so that you don't have to grow up.
Either this continues, feeling fake and unsatisfactory as a story - or the author begins having frustrations set in, until finally it all comes crashing down in a tragedy and the hero dies horribly while the world around them rejoices at his justly deserved death.
It all feels to me like kids to day are extremely alienated from others. Alienated from their families, their school mates, peers, everyone. Probably preferring their online persona over real life interactions.
Basically just a diluted but widespread form of Evercrack addiction, with the attendant side effects, just more dilluted so people don't notice right away.